1. Field of the Invention
The present invention relates to a vehicle generator control device for controlling the output voltage of an AC generator installed in a motor vehicle such as passenger cars, trucks or the like.
2. Description of the Related Art
In recent years there has been an increasing demand for the effective use of the output of a vehicle generator driven for rotation by an engine installed in a motor vehicle. To meet the demand, proposals have been made wherein a “transistor chopper” type exciter circuit is provided to use a rotor excitation current as a regenerative current to be supplied to the battery for the purpose of achieving an effective use of the generator output.
One example of such prior proposals is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 62-203599, which comprises a bridge circuit having a pair of opposing arms constituted by respective transistors. When the vehicle engine is in the non-operating state, the transistors of the bridge circuit are switched to the off state with the result that the field winding of a rotor of the vehicle generator is in a floating state from the viewpoint of electric potential. In cold climates, for example, it may occur that after traveling of the vehicle, the vehicle generator is left to stand as being wet with water from melted snow containing a snow-melting agent. In this instance, if a leakage current occurs between transistor terminals connectable to a battery terminal of the rotor, the rotor potential becomes equal to the battery potential, allowing the current to flow between the rotor and a stator core having ground potential. As a result, the rotor and the stator core, which face across a very narrow space so-called “air gap”, get rusty, deteriorating the environment resistance and reliability of the vehicle generator. This phenomenon becomes prominent especially when the vehicle generator is used at a high voltage (42 V, for example). Due to the foregoing difficulties, the vehicle generator equipped with a transistor chopper type exciter circuit has not come into widespread use, although it is multifunctional under normal conditions.